DESCRIPTION (Verbatim from Applicant's Abstract): The objective of this research is the development of new scintillators with very rapid lum aboutnescence decay, and high atomic number, density, and luminosity. These properties would improve the detection efficiency, deadtime, spatial resolution, and scatter and randoms rejection of positron tomographs. A timing resolution of +100 ps would increase the effective sensitivity by a factor of six for images of the human brain and thorax, and allow event-by-event slice determination in 3D tomography. Development of a low-cost scintillator with these properties would improve the availability and accuracy of PET for medical studies throughout the world. Experimental and computational work on the fast, bright scintillator ZnO:Ga has led us to understand its excitation and emission mechanisms and to propose the objective of achieving them in a crystal with good stopping power for PET. A second objective is the identification of heavy-atom crystals that would be good hosts for Ce3+ activation. To accomplish these objectives, we will continue to improve our methods for using embedded molecular orbital cluster and electronic band structure calculations for simulating luminescence mechanisms, and apply them first to known scintillators, and then to unexplored materials to guide the selection of candidates for synthesis and experimental measurement. Processes to be simulated include (1 ) the formation and transport of donor electrons, holes, and excitons, (2) lattice relaxation, (3) the ground and excited states of impurity atoms, and (4) thermal quenching. Candidate materials will be synthesized as powders and tested using our pulsed x-ray system.